Masters Theses

Date of Award

3-1981

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Lawrence A. Taylor

Committee Members

Otto C. Kopp

Abstract

Kimberlite from Elliott County, Kentucky, contains abundant metasomatized crustal materials, ultramafic xenoliths, and megacrysts in a diverse assemblage of silicates and oxides. A distinction has been made between "early-stage" and "late-stage" groundmass minerals. This study deals with the "late-stage" groundmass assemblage of phlogopite, vermiculite, pectolite, calcite, and septechlorite (4.55-11.8% Al2O3); perovskite and titano-magnetite may also belong to this assemblage. Halos of serpentinized olivines (Al-poor lizardite; 0.19-2.99% Al2O3) surround the crustal inclusions.

The most abundant inclusions are siliceous and argillaceous limestones, shales, and a variety of mafic and felsic crustal materials; many highly altered felsic crustal clasts are interpreted to have been fragments of sandstone, felsic granulite and calc-silicate rock. The crustal inclusions occur in the following relative abundances: shale > carbonate > mafic rock > calc-silicate rock > felsic granulites or sandstones. Carbonate clasts are extremely heterogeneous; they commonly contain detrital zircons and exhibit relict textures. These carbonate clasts contain the assemblage calcite + septechlorite + phlogopite + wollastonite diopside mixed-layer mineral. Many calc-silicate clasts have central cores completely converted to pectolite with distinct reaction zones of wollastonite + diopside; others are completely converted to thomsonite + Ti-rich hydrogarnet. Mafic clasts have cores of hydrous Mg-silicates (lizardite, septechlorite, or mixed-layer minerals) + Ti-rich hydrogarnet. These clasts are separated from the host kimberlite by a zone of septechlorite ± phlogopite ± calcite; toward the clast core, a zone of diopside ± actinolite ± pectolite is commonly present. All shale clasts are separated from the host kimberlite by a zone of phlogopite + septechlorite.

Electron microprobe defocused beam analyses of metasomatic zones indicate major element chemical gradients across the alteration zones as manifest by the distinct mineral assemblages of the zones. In shale and calc-silicate clasts, SiO2, CaO, and Na2O decrease toward the contact with kimberlite; in mafic clasts, Al2O3 and Cao increase outward. TiO2 is enriched in the cores of mafic clasts and the thomsonite-bearing calc-silicate clasts; K2O and MgO enrichment at the contact of clasts with kimberlite is manifest as the assemblage phlogopite + septechlorite.

These mineral assemblages constrain the P-T-X conditions during the final stages of the evolution of the Elliott County kimberlite. "Late-stage" groundmass Al-poor septechlorites have maximum thermal stabilities of 685°C. If serpentinization of the groundmass olivines is associated with the metasomatic alteration of the clasts, the occurrence of Al-poor lizardite, and Al-poor septechlorite within the reaction zones constrains the XC)2 ≤ 0.10. At this value of XCO2 the assemblages wollastonite + diopside and wollastonite + calcite suggest T ~ 400°-525°C at P = 500 bars. The presence of thomsonite indicates T ≤ 350°C and XC02 ≤ 0.02. These P-T-X estimates suggest the formation of the late-stage kimberlite groundmass minerals and the metasomatic alteration of crustal clasts occurred over a range of T and XCO2.

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